The Baeyer–Villiger oxidation is an organic reaction that forms an ester from a ketone or a lactone from a cyclic ketone, using peroxyacids or peroxides as the oxidant. The reaction is named after Adolf von Baeyer and Victor Villiger, who first reported the reaction in 1899.

In the first step of the reaction mechanism, the peroxyacid protonates the oxygen of the carbonyl group. This makes the carbonyl group more susceptible to be attacked by the peroxyacid. Next, the peroxyacid attacks the carbon of the carbonyl group forming what is known as the Criegee intermediate. Through a concerted mechanism, one of the substituents on the ketone group migrates to the oxygen of the peroxide group while a carboxylic acid leaves. This migration step is thought to be the rate determining step. Finally, deprotonation of the oxocarbenium ion produces the ester.

The products of the Baeyer–Villiger oxidation are believed to be controlled through both primary and secondary stereoelectronic effects. The primary stereoelectronic effect in the Baeyer–Villiger oxidation refers to the necessity of the oxygen-oxygen bond in the peroxide group to be antiperiplanar to the group that migrates. This orientation facilitates optimum overlap of the 𝛔 orbital of the migrating group to the 𝛔* orbital of the peroxide group. The secondary stereoelectronic effect refers to the necessity of the lone pair on the oxygen of the hydroxyl group to be antiperiplanar to the migrating group. This allows for optimum overlap of the oxygen nonbonding orbital with the 𝛔* orbital of the migrating group. This migration step is also (at least in silico) assisted by two or three peroxyacid units enabling the hydroxyl proton to shuttle to its new position.

The migratory ability is ranked tertiary > secondary > aryl > primary. Allylic groups are more apt to migrate than primary alkyl groups but less so than secondary alkyl groups. Electron-withdrawing groups on the substituent decrease the rate of migration. There are two explanations for this trend in migration ability. One explanation relies on the buildup of positive charge in the transition state for breakdown of the Criegee intermediate (illustrated by the carbocation resonance structure of the Criegee intermediate). Keeping this structure in mind, it makes sense that the substituent that can maintain positive charge the best would be most likely to migrate. The higher the degree of substitution, the more stable a carbocation generally is. Therefore, the tertiary > secondary > primary trend is observed.

Another explanation uses stereoelectronic effects and steric arguments. As mentioned, the substituent that is antiperiplanar to the peroxide group in the transition state will migrate. This transition state has a gauche interaction between the peroxyacid and the non-migrating substituent. If the bulkier group is placed antiperiplanar to the peroxide group, the gauche interaction between the substituent on the forming ester and the carbonyl group of the peroxyacid will be reduced. Thus, it is the bulkier group that will prefer to be antiperiplanar to the peroxide group, enhancing its aptitude for migration.

The migrating group in acyclic ketones, usually, is not 1° alkyl group. However, they may be persuaded to migrate in preference to the 2° or 3° groups by using CF₃CO₃H or BF₃ + H₂O₂ as reagents.

The migration does not change the stereochemistry of the group that transfers, i.e.: it is stereoretentive.

In 1899, Adolf Baeyer and Victor Villiger demonstrated the reaction now known as the Baeyer–Villiger oxidation. They used peroxymonosulfuric acid to make the corresponding lactones from camphor, menthone, and tetrahydrocarvone.